Dual passband radio frequency filter and communications device

ABSTRACT

Multi-band filters, communications devices, and methods of designing multi-band filters are disclosed. A multi-band filter has a lower pass-band and an upper pass-band separated by an intervening stop-band. The multi-band filter includes a first ladder network and a second ladder network coupled in series. The first ladder network provides transmission zeros at frequencies below a lower edge of the lower pass-band and transmission zeros at frequencies above an upper edge of the upper pass-band. The second ladder network provides transmission zeros at frequencies within the intervening stop-band.

RELATED APPLICATION INFORMATION

This patent is a continuation of patent application Ser. No. 15/485,413,filed Apr. 12, 2017, titled DUAL PASSBAND RADIO FREQUENCY FILTER ANDCOMMUNICATIONS DEVICE, now U.S. Pat. No. 9,825,611 B2, which claimspriority from provisional patent application 62/323,414, filed Apr. 15,2016, titled MULTI-PASSBAND FILTERS FOR MOBILE DEVICE RF FRONT ENDS,which are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using surfaceacoustic wave (SAW) resonators, and specifically to filters for use incommunications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-terminal device configured topass some frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low insertion loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is less than adefined value such as one dB, two dB, or three dB. A “stop-band” may bedefined as a frequency range where the insertion loss of a filter isgreater than a defined value such as twenty dB, twenty-five dB, fortydB, or greater depending on application.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of base stations, mobile telephone and computingdevices, satellite transceivers and ground stations, IoT (Internet ofThings) devices, laptop computers and tablets, fixed point radio links,and other communications systems. RF filters are also used in radar andelectronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between such performanceparameters as insertion loss, rejection, isolation, power handling,linearity, size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

Surface acoustic wave (SAW) resonators are used in a variety of RFfilters including band-reject filters, band-pass filters, duplexers, andmultiplexers. A duplexer is a radio frequency filter device that allowssimultaneous transmission in a first frequency band and reception in asecond frequency band (different from the first frequency band) using acommon antenna. A multiplexer is a radio frequency filter with more thantwo input or output ports with multiple pass-bands. A triplexer is afour-port multiplexer with three pass-bands.

As shown in FIG. 1, a typical SAW resonator 100 is formed by thin filmconductor patterns formed on a surface of a substrate 105 made of apiezoelectric material such as quartz, lithium niobate, lithiumtantalate, or lanthanum gallium silicate. The substrate 105 is commonlya single-crystal slab of the piezoelectric material, or a compositesubstrate including a thin single-crystal wafer of the piezoelectricmaterial bonded to another material such as silicon, sapphire, orquartz. A composite substrate is commonly used to provide a thermalexpansion coefficient different from the thermal expansion coefficientof the single-crystal piezoelectric material alone. A firstinter-digital transducer (IDT) 110 includes a plurality of parallelconductors. A radio frequency or microwave signal applied to the firstIDT 110 via an input terminal IN generates an acoustic wave on thesurface of the substrate 105. As shown in FIG. 1, the surface acousticwave will propagate in the left-right direction. A second IDT 120converts the acoustic wave back into a radio frequency or microwavesignal at an output terminal OUT. The conductors of the second IDT 120are interleaved with the conductors of the first IDT 110 as shown. Inother typical SAW resonator configurations (not shown), the conductorsforming the second IDT are disposed on the surface of the substrate 105adjacent to, or separated from, the conductors forming the first IDT.Also, extra fingers (commonly called “dummy” fingers) are sometimesformed opposite to the ends of the IDT fingers and connected to the INand OUT bus bars of the first and second IDTs 110 and 120. Gratingreflectors 130, 135 are disposed on the substrate to confine most of theenergy of the acoustic waves to the area of the substrate occupied bythe first and second IDTs 110, 120. The grating reflectors 130, 135float or are connected to either the IN terminal or the OUT terminal. Ingeneral, the SAW resonator 100 is bi-directional, and the IN and OUTterminal designations may be transposed.

The electro-acoustic coupling between the first IDT 110 and the secondIDT 120 is highly frequency-dependent. The basic behavior of acousticresonators (SAW, bulk acoustic wave, film bulk acoustic wave, etc.) iscommonly described using the Butterworth Van Dyke (BVD) circuit model asshown in FIG. 2A. The BVD circuit model consists of a motional arm and astatic arm. The motional arm includes a motional inductance L_(m), amotional capacitance C_(m), and a resistance R_(m). The static armincludes a static capacitance C₀ and a resistance R₀. While the BVDmodel does not fully describe the behavior of an acoustic resonator, itdoes a good job of modeling the two primary resonances that are used todesign band-pass filters, duplexers, and multiplexers (multiplexers arefilters with more than 2 input or output ports with multiplepass-bands).

The first primary resonance of the BVD model is the motional resonancecaused by the series combination of the motional inductance L_(m) andthe motional capacitance C_(m). The second primary resonance of the BVDmodel is the anti-resonance caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the motional resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$where γ=C₀/C_(m) is a characteristic of the substrate upon which the SAWresonator is fabricated. γ is dependent on both the material and theorientation of the crystalline axes of the substrate, as well as thephysical design of the IDTs.

The frequencies of the motional resonance and the anti-resonance aredetermined primarily by the pitch and orientation of the interdigitatedconductors, the choice of substrate material, and the crystallographicorientation of the substrate material.

FIG. 2B is a plot of the admittance of a theoretical lossless acousticresonator. The admittance exhibits a motional resonance 212 where theadmittance of the resonator approaches infinity, and an anti-resonance214 where the admittance of the resonator approaches zero. Inover-simplified terms, the lossless acoustic resonator can be considereda short circuit at the frequency of the motional resonance 212 and anopen circuit at the frequency of the anti-resonance 214. The frequenciesof the motional resonance 212 and the anti-resonance 214 arerepresentative, and a resonator may be designed for other frequencies.

Cellular telephones operate in various bands defined by industry orgovernmental standards. For example, the 3GPP LTE (Third GenerationPartnership Project Long Term Evolution) standard defines 48 differentbands over a frequency range of about 450 MHz to greater than 5000 MHz.Each of these bands consists of a frequency range or a pair of disjointfrequency ranges used for cellular telephone communications. Forexample, Band 12, which is used in the United States and Canada, employsthe frequency range from 699 MHz to 716 MHz for communications from thecellular device to the cellular network and the frequency range from 729MHz to 746 MHz for communications from the network to the device. Band40, used in several countries in Asia, employs the frequency range from2300 MHz to 2400 MHz for communications in both directions. All of bandsdefined by the 3GPP LTE standard are not currently in use, and only oneor a few bands are typically used in any particular country. Further,different cellular service providers in a given country may each havefrequency allocations within one or multiple bands.

Antenna diversity uses two or more antennas to improve the quality andreliability of a wireless link. In urban and indoor environments, atransmitted signal may be reflected along multiple paths before finallybeing received. Each of these reflections introduces a phase shift ortime delay Such that signals reflected along differ paths maydestructively interfere with one another at the receiving antenna. Thissituation, commonly called “multipath fading,” can result incommunications drop-outs.

The use of multiple, physically separated, antennas on a communicationsdevice is an effective technique for mitigating multipath fading. Eachantenna will experience a different interference environment. If oneantenna is subject to severe multipath fading, it is likely that asecond antenna disposed a short distance from the first antenna willhave a sufficient signal. Current communications devices, such ascellular telephones, typically include a second antenna and a secondreceiver chain to improve call reliability. The second antenna andreceiver chain are commonly referred to as a “diversity receiver.”

In order to increase data rate, a technique called carrier aggregationmay also be used. Carrier aggregation uses multiple bands simultaneouslyfor the transmission and reception of data, thereby increasing the datathroughput rate. When receiving data transmitted using carrieraggregation, it may be advantageous for the receiver to include a singlefilter with multiple pass-bands matching the multiple bands beingtransmitting.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic plan view of a SAW resonator.

FIG. 2A is an equivalent circuit of a SAW resonator.

FIG. 2B is graph of the admittance of a lossless SAW resonator.

FIG. 3 is a schematic diagram of a conventional SAW filter.

FIG. 4 is a schematic diagram of a dual band-pass SAW filter.

FIG. 5 is a chart showing the input-output transfer function of ahypothetical dual band-pass SAW filter.

FIG. 6 is a schematic diagram of a Band 2/Band 4 dual band-pass SAWfilter.

FIG. 7 is a chart showing the input-output transfer function of the Band2/Band 4 dual band-pass SAW filter.

FIG. 8 is a schematic diagram of a Band1/Band3 dual band-pass SAWfilter.

FIG. 9 is a chart showing the input-output transfer function of the Band1/Band 3 dual band-pass SAW filter.

FIG. 10 is a schematic diagram of a three-band SAW filter.

FIG. 11 is a chart showing the input-output transfer function of thethree-band SAW filter.

FIG. 12A is a block diagram of a conventional RF front end.

FIG. 12B is a block diagram of another conventional RF front end.

FIG. 12C is a block diagram of an RF front end including a dualband-pass filter.

FIG. 13 is a flow chart of a method for designing a dual band-pass SAWfilter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 3 shows a schematic diagram of an exemplary band-pass filtercircuit 300 incorporating eleven SAW resonators, labeled X1 through X11,arranged in what is commonly called a “ladder” configuration. The filtercircuit 300 may be, for example, a transmit filter or a receive filterfor incorporation into a communications device. The filter circuit 300includes six series resonators (X1, X3, X5, X7, X9, and X11) connectedin series between an input (Port 1) and an output (Port 2). The filtercircuit 300 includes five shunt resonators (X2, X4, X6, X8, and X10)connected between junctions of adjacent series resonators and ground.Shunt resonators may also be connected between ground and either or bothof the input port and the output port. The use of eleven SAW resonators,six series resonators, and five shunt resonators is exemplary. A filtercircuit may include more or fewer than eleven SAW resonators and adifferent arrangement of series and shunt resonators. Although not shownin FIG. 3 or any of the other schematic diagrams in this patent, filtercircuits may also include reactive components such as inductors andcapacitors.

Each of the eleven resonators X1-X11 may be comprised of inter-digitaltransducers and grating reflectors as shown in FIG. 1. Each of theeleven resonators X1-X11 may have a corresponding motional resonantfrequency, F1-F11. The motional resonant frequencies F1-F11 may all bedifferent. The motional resonant frequencies of some of the resonatorsX1-X11 may be the same. Typically, the motional resonant frequencies F2,F4, F6, F8, F10 of the shunt resonators are offset from the motionalresonant frequencies F1, F3, F5, F7, F9, F11 of the series resonators.

Assuming the resonators X1-X11 are lossless, each shunt resonator X2,X4, X6, X8, and X10 acts as a short circuit at its respective motionalresonant frequency, and each series resonator X1, X3, X5, X7, X9, andX11 acts as an open circuit at its respective anti-resonant frequency.Considering the filter circuit 300 as a whole, it can be understood thatthe attenuation of the filter from input to output is infinite (underthe assumption that the resonators are lossless) at the motionalresonant frequencies of the shunt resonators and the anti-resonantfrequencies of the series resonators. These frequencies are commonlycalled attenuation poles or, equivalently, transmission zeros. Thepass-band of the filter circuit 300 is a frequency range between thehigh and low side transmission zeros. In a typical ladder band-passfilter design, the motional resonant frequencies of series resonatorsfall within a pass-band frequency range, the anti-resonant frequenciesof series resonators create transmission zeros above the pass-bandfrequency range. The opposite is true for shunt resonators. Theanti-resonant frequencies of shunt resonators are within the pass-bandfrequency range, and the motional resonant frequencies of shuntresonators create transmission zeros below the pass-band frequencyrange.

FIG. 4 shows a schematic diagram of an exemplary dual band-pass filtercircuit 400 including a first ladder network 410 in series with a secondladder network 420. In this example, the first ladder network 410includes three series resonators X1, X3, X5, and three shunt resonatorsX2, X4, X6. The second ladder network includes three series resonatorsX7, X9, X11, and two shunt resonators X8, X10. Each of the elevenresonators X1-X11 may be comprised of inter-digital transducers andgrating reflectors as shown in FIG. 1. Each of the eleven resonatorsX1-X11 has a corresponding motional resonant frequency, F1-F11. Eachshunt resonator X2, X4, X6, X8, and X10 causes a transmission zero atits respective motional resonant frequency, and each series resonatorX1, X3, X5, X7, X9, and X11 causes a transmission zero at its respectiveanti-resonant frequency.

FIG. 5 is a graph 500 of the S(2,1) parameter of a hypotheticalembodiment of the dual pass-band filter 400. S-parameters are aconvention used to describe the performance of linear electricalnetworks. The solid line 510 is a plot of S(2,1), which is the voltagetransfer function from port 1 to port 2 of the dual pass-band filter.S(2,1) is often expressed in dB, which is 20 log₁₀ [S(2,1)], and isessentially the power gain of the device. However, passive devices likeSAW filters are usually characterized by the “insertion loss” of thefilter, which is numerically the same as the power gain, but with achange in numeric sign (e.g. S(2,1)=−3 dB is equivalent to an insertionloss of 3 dB). In this case, the solid line 510 plots theinput-to-output transfer function of the filter 400. As shown, thefilter 400 has low insertion loss in a lower pass-band and an upperpass-band, where “lower” and “upper” refer to the relative frequenciesof the pass-bands. The lower and upper pass-bands are separated by anintervening stop-band, where an “intervening stop band” is a stop bandthat lies between two passbands.

Also shown in the graph 500 are the frequency locations of transmissionzeros created by either the motional resonance of shunt resonators, orthe anti-resonance of series resonators. The transmission zero frequencylocations as indicated by dashed and broken arrows extending downwardfrom the top of the graph 500. These arrows are shown for convenience inunderstanding the shape of the transfer function, and are not part ofthe measured or simulated response of the dual pass-band filter 400.

Three dashed arrows 520 located below the lower edge of the lowerpass-band represent transmission zeros caused by the motional resonancesof the shunt resonators X2, X4, X6 in the first ladder network 410.Three broken (dash-dot) arrows 530 located above the upper edge of theupper pass-band represent transmission zeros caused by theanti-resonances of series resonators X1, X3, X5 in the first laddernetwork. Five broken (dash-dot-dot) arrows 540 located within theintervening stop band represent transmission zeros caused by theresonators X7 to X11 in the second ladder network. Typically,transmission zeros caused by the series resonators of the second laddernetwork are located near the lower side of the intervening stop-band(just above the upper edge of the lower pass-band) and transmissionzeros caused by the shunt resonators of the second ladder network arelocated in the upper half of the intervening stop band (just below thelower edge of the upper pass-band).

The arrangement of series and shunt SAW resonators shown in FIG. 4 isexemplary. Each of the first and second ladder networks may include moreor fewer SAW resonators and a different arrangement of series and shuntresonators. In all cases, shunt resonators in the first ladder networkcreate transmission zeros at frequencies below the lower edge of thelower pass-band, series resonators in the first ladder network createtransmission zeros at frequencies above the upper edge of the upperpass-band, and resonators in the second ladder network createtransmission zeros within the intervening stop-band.

Example 1

FIG. 6 shows a schematic diagram of band 2/band 4 dual band-pass filter600 including a first ladder network 610 in series with a second laddernetwork 620. The filter 600 has a lower pass-band and an upper pass-bandseparated by an intervening stop band. The lower pass-band accommodatesthe LTE band 2 receive band from 1930 to 1990 MHz. The upper pass-bandaccommodates the LTE band 4 receive band from 2110 to 2155 MHz. LTEbands 2 and 4 are widely used for cellular communications in North andSouth America, but are merely exemplary of relevant bands. The systemmay be applied to different frequency bands with similar effect. In FIG.6, the first ladder network 610 includes four shunt resonators X1, X3,X5, X7, and four series resonators X2, X4, X6, X8. The second laddernetwork includes three shunt resonators X9, X11, X13, and two seriesresonators X10, X12. Each of the thirteen resonators X1-X13 may becomprised of inter-digital transducers and grating reflectors as shownin FIG. 1. The motional resonance frequency Fr and the staticcapacitance C₀ is provided for each resonator.

FIG. 7 is a graph 700 of the S(2,1) parameter of the band 2/band 4 dualpass-band filter 600. The solid line 710 is a plot of S(2,1), which isthe voltage transfer function from port 1 to port 2 of the dualpass-band filter 700.

Also shown in the graph 700 are the frequency locations of transmissionzeros created by either the motional resonance of shunt resonators, orthe anti-resonance of series resonators. The transmission zero frequencylocations as indicated by dashed and broken arrows extending downwardfrom the top of the graph 700. These arrows are shown for convenience inunderstanding the shape of the transfer function, and are not part ofthe measured or simulated response of the dual pass-band filter 700.

Four dashed arrows located below the lower edge of the lower pass-bandrepresent transmission zeros caused by the motional resonances of theshunt resonators X1, X3, X5, X7 in the first ladder network 610. Fourbroken (dash-dot) arrows located above the upper edge of the upperpass-band represent transmission zeros caused by the anti-resonances ofseries resonators X2, X4, X6, X8 in the first ladder network 610. Fivebroken (dash-dot-dot) arrows located within the intervening stop bandrepresent transmission zeros caused by the resonators X9 to X13 in thesecond ladder network 620. As is typical, transmission zeros caused bythe series resonators X10, X12 of the second ladder network 720 arelocated near the lower side of the intervening stop-band (just above theupper edge of the lower pass-band) and transmission zeros caused by theshunt resonators X9, X11, X13 of the second ladder 620 network arelocated in the upper half of the intervening stop band (just below thelower edge of the upper pass-band).

Example 2

FIG. 8 shows a schematic diagram of band 1/band 3 dual band-pass filter800 including a first ladder network 810 in series with a second laddernetwork 820. The filter 800 has a lower pass-band and an upper pass-bandseparated by an intervening stop band. The lower pass-band accommodatesthe LTE band 3 receive band from 1805 to 1880 MHz. The upper pass-bandaccommodates the LTE band 1 receive band from 2110 to 2170 MHz. LTEbands 1 and 3 are widely used for cellular communications in Asia andEurope. The first ladder network 810 includes five series resonators X1,X3, X5, X7, X9 and four shunt resonators X2, X4, X6, X8. The secondladder network includes three shunt resonators X10, X12, X15, and threeseries resonators X11, X13, X15. Each of the fifteen resonators X1-X15may be comprised of inter-digital transducers and grating reflectors asshown in FIG. 1. The motional resonance frequency Fr and the staticcapacitance C₀ is provided for each resonator.

FIG. 9 is a graph 900 of the S(2,1) parameter of the band 1/band 3 dualpass-band filter 800. The solid line 910 is a plot of S(2,1), which isthe voltage transfer function from port 1 to port 2 of the dualpass-band filter 800.

Also shown in the graph 900 are the frequency locations of transmissionzeros created by either the motional resonance of shunt resonators, orthe anti-resonance of series resonators. The transmission zero frequencylocations as indicated by dashed and broken arrows extending downwardfrom the top of the graph 900. These arrows are shown for convenience inunderstanding the shape of the transfer function, and are not part ofthe measured or simulated response of the dual pass-band filter 800.

Four dashed arrows located below the lower edge of the lower pass-bandrepresent transmission zeros caused by the motional resonances of theshunt resonators X2, X4, X6, X8 in the first ladder network 810. Fivebroken (dash-dot) arrows located above the upper edge of the upperpass-band represent transmission zeros caused by the anti-resonances ofseries resonators X1, X3, X5, X7, X9 in the first ladder network 810.Six broken (dash-dot-dot) arrows located within the intervening stopband represent transmission zeros caused by the resonators X10 to X15 inthe second ladder network 820. As is typical, transmission zeros causedby the series resonators X11, X13, X15 of the second ladder network 820are located near the lower side of the intervening stop-band (just abovethe upper edge of the lower pass-band) and transmission zeros caused bythe shunt resonators X10, X112, X14 of the second ladder 820 network arelocated in the upper half of the intervening stop band (just below thelower edge of the upper pass-band).

Example 3

FIG. 10 shows a schematic diagram of an exemplary three-band filter 1000including a first ladder network 1010 and a second ladder network 1020in series between an input port (Port 1) and an output port (Port 2).The first ladder network 1010 includes three series resonators X1, X3,X5 and three shunt resonators X2, X4, X6. The second ladder network 1020includes four series resonators X7, X9, X11, X13 and four shuntresonators X8, X10, X12, X14. Each of the fourteen resonators X1-X14 maybe comprised of inter-digital transducers and grating reflectors asshown in FIG. 1. The motional resonance frequency Fr and the staticcapacitance C₀ is provided for each resonator.

FIG. 11 is a graph 1100 of the S(2,1) parameter of the three-band filter1000. The solid line 1010 is a plot of S(2,1), which is the voltagetransfer function from port 1 to port 2 of the three-band filter 1000.The filter 1000 has a lower pass-band, a middle pass-band, and an upperpass-band separated by lower and upper intervening stop bands.

Also shown in the graph 1100 are the frequency locations of transmissionzeros created by either the motional resonance of shunt resonators, orthe anti-resonance of series resonators. The transmission zero frequencylocations as indicated by dashed and broken arrows extending downwardfrom the top of the graph 1100. These arrows are shown for conveniencein understanding the shape of the transfer function, and are not part ofthe measured or simulated response of the three-band filter 1000.

Three dashed arrows located below the lower edge of the lower pass-bandrepresent transmission zeros caused by the motional resonances of theshunt resonators X2, X4, X6, in the first ladder network 1010. Threebroken (dash-dot) arrows located above the upper edge of the upperpass-band represent transmission zeros caused by the anti-resonances ofseries resonators X1, X3, X5, in the first ladder network 1010. Eightbroken (dash-dot-dot) arrows located within the lower and upperintervening stop band represent transmission zeros caused by theresonators X7 to X14 in the second ladder network 1020. Specifically,transmission zeros caused by the series resonators X11, X13 are locatednear the lower side of the lower intervening stop-band (just above theupper edge of the lower pass-band) and transmission zeros caused by theshunt resonators X8, X10 are located in the upper half of the lowerintervening stop band (just below the lower edge of the middlepass-band). Transmission zeros caused by the series resonators X7, X9are located near the lower side of the upper intervening stop-band (justabove the upper edge of the middle pass-band) and transmission zeroscaused by the shunt resonators X12, X14 are located in the upper half ofthe upper intervening stop band (just below the lower edge of the upperpass-band).

FIG. 12A is a block diagram of a portion of a diversity receiver 1210for a communications device. In this example, the diversity receiverincludes a first RF switch 1214 that directs a signal from an antenna1212 to a selected one of a Band A filter 1216 and a Band B filter 1218,where “Band A” and “Band B” may be any two of the numbered LTE bands. Asecond RF switch 1220 directs the output from the selected filter to alow noise amplifier (LNA) 1222.

FIG. 12B is a block diagram of a portion of another diversity receiver1230 for a communications device. In this example, the diversityreceiver includes a first RF switch 1234 that directs a signal from anantenna 1212 to a selected one of a Band A filter 1236 and a Band Bfilter 1238, where “Band A” and “Band B” may be any two of the numberedLTE bands. The outputs from the filters 1236, 1238 are amplified byrespective LNAs 1240, 1242.

FIG. 12C is a block diagram of a portion of an improved diversityreceiver 1250 for a communications device. The improved diversityreceiver includes an antenna 1252 coupled to an LNA 1256 through adual-band filter 1254. The dual-band filter 1254 may be, for example,the Band 2/Band 4 filter 600 of FIG. 6 or the Band 1/Band 3 filter 800of FIG. 8.

Comparison FIG. 12C with FIG. 12A and FIG. 12B shows that the improveddiversity receiver 1250 requires fewer components and thus has apotential for reduced cost compared to the prior art diversity receivers1210, 1230. Further, since RF switches are not required, the improveddiversity receiver 1250 offers improved throughput between the antennaand LNA and corresponding higher signal-to-noise ratio at the input tothe LNA. The receiver may also be used for carrier aggregation, wheretwo receive bands are processed simultaneously, to increase thebandwidth used, thereby increasing the data rate of the system.

Description of Methods

FIG. 13 is a flow chart of a process 1300 for designing a multi-bandfilter. The process 1300 starts at 1305 with a specification on thefilter to be designed. The process 1300 ends at 1395 upon completion ofa satisfactory filter design. The process 1300 may be, in many cases,cyclic, with various steps of the process repeated iteratively to arriveat a satisfactory design.

The specification at 1305 may define both required and desiredcharacteristics of the filter. Typically, the specification on amulti-band bandpass filter will define frequency ranges for a lowerpass-band, an upper pass-band, and one or more stopband frequencyranges. The specification may define frequency ranges for one or moreintermediate pass bands between the lower pass-band and theupper-pass-band. The specification may also define a maximum insertionloss over each passband frequency range, and/or a minimum insertion lossfor each of the stopband frequency ranges. The specification may includeother parameters such as a minimum return loss or maximum VSWR (voltagestanding wave ratio) at the input and/or output of the filter over thepassband frequency range when the filter is coupled to a particularimpedance.

At 1310, a set of design objectives may be established based on thespecification from 1305. Typically, the set of design objectives willinclude all of the required characteristics from the specification aswell as targets for any desired characteristics. The set of designobjectives may also include cost-driven targets such as a maximum numberof resonators, a particular type of piezoelectric substrate, particularmanufacturing processes, and/or a maxim piezoelectric substrate area.

At 1320, a first ladder network may be defined as an attempt to satisfythe design objectives from 1310. The definition of the first laddernetwork may include quantities of shunt resonators and series resonatorsand initial values for the motional resonant frequency or anti-resonantfrequency of each resonator Specifically, initial values of the motionalresonant frequencies of shunt resonators may be defined to providetransmission zeros below the lower edge of the lower pass-band, andinitial values of the anti-resonant frequencies of series resonators maybe defined to provide transmission zeros above the upper edge of theupper pass-band. The definition of the first ladder network at 1320 maybe performed, for example, by a design engineer, possibly with theassistance of a software tool or “wizard” that suggests a number ofresonators and motional resonant/anti-resonant frequencies based uponthe design objectives from 1310.

At 1330, a second ladder network may be defined as an attempt to satisfythe design objectives from 1310. The definition of the second laddernetwork may include quantities of shunt resonators and series resonatorsand initial values for the motional resonant frequency or anti-resonantfrequency of each resonator. Specifically, initial values of themotional resonant frequencies of shunt resonators and initial values ofthe anti-resonant frequencies of series resonators may be defined toprovide transmission zeros within one or more stop-bands intermediatethe upper and lower pass-bands. The definition of the second laddernetwork at 1330 may be performed, for example, by a design engineer,possibly with the assistance of a software tool if available.

Starting with the definitions of the first ladder network and secondladder network from 1320 and 1330, the overall design of the multi-bandfilter is optimized at 1340. The design optimization at 1340 may beperformed, for example, by the design engineer using a circuit designsoftware tool and/or an electroacoustic (EA) analysis software tool.When a circuit design tool is used, the ladder filter is analyzed as anelectronic circuit, with the SAW resonators represented by combinationsof lumped capacitor, inductor, and resistor elements. For example, eachresonator may be represented by a BVD model as previously shown in FIG.2A. When an EA analysis tool is used, the filter is represented bymodels of the SAW resonator IDTs on a piezoelectric substrate. Apreliminary design may be performed using a circuit analysis tool andthen refined using an EA analysis tool. Either or both of circuit designtool and the EA analysis tool may be capable of automated optimizationof the filter design to satisfy, to the extent possible, the designobjectives. Although real resonators have loss, an initial filter designoptimization at 1340 may be done assuming lossless resonators, with theloss effects added later in the design effort.

At 1350, a determination is made whether or not the optimized filterdesign from 1340 is acceptable, which is to say whether or not thefilter design from 1340 satisfies (or is acceptably close to satisfying)the design objectives from 1310. When the filter design is satisfactory(“yes” at 1350), the design may be completed at 1360. Completing thedesign at 1360 may include, for example, defining a package for thefilter and necessary interconnections within the package. Completing thedesign may also include adjusting the filter design if necessary tocompensate for the effects (e.g. interconnection inductances and straycapacitances) of the package and interconnects. Once the design iscompleted at 1360, the process ends at 1395.

When a determination is made at 1350 that the filter design from 1340 isnot acceptable (“no” at 1350), the process may return to 1320. Thedefinitions of one or both the first ladder network and the secondladder network may be modified (for, example by adding one or moreadditional resonators) and the actions from 1320 to 1350 may be repeateduntil an acceptable filter design is established. In some circumstances,for example after multiple iterations of the actions from 1320 to 1350,the process may return to 1310 to re-establish the design objective forthe multi-band filter. For example, the design objectives may bemodified to allow a larger piezoelectric substrate area to accommodatemore resonators or a different type of piezoelectric substrate toprovide different resonator performance.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A communications device, comprising: a multi-bandfilter having a lower pass-band and an upper pass-band separated by anintervening stop-band, comprising: a first plurality of acoustic waveresonators including two or more first-network shunt resonators and twoor more first-network series resonators coupled to form a first laddernetwork; and a second plurality of acoustic wave resonators forming asecond ladder network, the first and second ladder networks coupled inseries between a first port and a second port, wherein the two or morefirst-network shunt resonators have respective motional resonancefrequencies below a lower edge of the lower passband to providetransmission zeros below the lower edge of the lower pass-band and thetwo or more first-network series resonators have respectiveanti-resonance frequencies above an upper edge of the upper passband toprovide transmission zeros above the upper edge of the upper pass-band,and the second ladder network is configured to provide transmissionzeros at frequencies within the intervening stop-band.
 2. Thecommunications device of claim 1, wherein the second plurality ofacoustic wave resonators comprises: two or more second-network shuntresonators and two or more second-network series resonators, whereinrespective motional resonance frequencies of each of the second-networkshunt resonators and respective anti-resonance frequencies of each ofthe second network series resonators are within the intervening stopband.
 3. The communications device of claim 2, wherein the interveningstop-band is divided into a lower stop-band and an upper stop-bandseparated by a middle pass-band, and respective motional resonancefrequencies of each of the second-network shunt resonators andrespective anti-resonance frequencies of each of the second networkseries resonators fall within one of the lower stop-band and the upperstop band.
 4. The communications device of claim 2, wherein each of thefirst-network series resonators, the first-network shunt resonators, thesecond-network series resonators, and the second-network shuntresonators is a surface acoustic wave (SAW) resonator.
 5. Thecommunications device of claim 1, wherein the multi-band filter is aBand 2/Band 4 filter having a lower pass-band of 1930 MHz to 1990 MHzand an upper pass-band of 2100 MHz to 2155 MHz, the first plurality ofacoustic wave resonators comprises four shunt resonators and four seriesresonators, and the second plurality of acoustic wave resonatorscomprises three shunt resonators and two series resonators.
 6. Thecommunications device of claim 1, wherein the multi-band filter is aBand 1/Band 3 filter having a lower pass-band of 1805 MHz to 1880 MHzand an upper pass-band of 2100 MHz to 2170 MHz, the first plurality ofacoustic wave resonators comprises four shunt resonators and five seriesresonators, and the second plurality of acoustic wave resonatorscomprises three shunt resonators and three series resonators.
 7. Thecommunications device of claim 1, further comprising: an antenna coupledto the first port of the multi-band filter; and a low noise amplifiercoupled to the second port of the multi-band filter.
 8. Thecommunications device of claim 7, wherein the antenna, the multi-bandfilter, and the low noise amplifier are a portion of a diversityreceiver within a cellular telephone.
 9. A method of designing amulti-band filter having a lower pass-band and an upper pass-bandseparated by an intervening stop-band, the method comprising: defining afirst plurality of acoustic wave resonators including two or morefirst-network shunt resonators and two or more first-network seriesresonators coupled to form a first ladder network to providetransmission zeros at frequencies below a lower edge of the lowerpass-band and transmission zeros at frequencies above an upper edge ofthe upper pass-band; and defining a second plurality of acoustic waveresonators forming a second ladder network to provide transmission zerosat frequencies within the intervening stop-band, wherein the firstladder network and the second ladder network are coupled in seriesbetween a first port and a second port, and defining the first pluralityof acoustic wave resonators further comprises: selecting respectivemotional resonance frequencies of the first-network shunt resonatorsbelow the lower edge of the lower pass-band; and selecting respectiveanti-resonance frequencies for the first-network series resonators abovethe upper edge of the upper pass-band.
 10. The method of designing amulti-band filter of claim 9, wherein defining the second plurality ofacoustic wave resonators comprises: defining-two or more second-networkshunt resonators and two or more second-network series resonatorscoupled to form the second ladder network; and selecting respectivemotional resonance frequencies of the second-network shunt resonatorsand respective anti-resonance frequencies of the second-network seriesresonators within the intervening stop band.
 11. The method of designinga multi-band filter of claim 10, wherein the respective motionalresonance frequencies of the second-network shunt resonators andrespective anti-resonance frequencies of the second-network seriesresonators are selected such that the intervening stop-band is dividedinto a lower stop-band and an upper stop-band separated by a middlepass-band.
 12. The method of designing a multi-band filter of claim 10,wherein each of the first-network series resonators, the first-networkshunt resonators, the second-network series resonators, and thesecond-network shunt resonators is a surface acoustic wave (SAW)resonator.
 13. The method of designing a multi-band filter of claim 9,wherein the multi-band filter is a Band 2/Band 4 filter having a lowerpass-band of 1930 MHz to 1990 MHz and an upper pass-band of 2100 MHz to2155 MHz, defining the first plurality of acoustic wave resonatorscomprises defining four first network shunt resonators having motionalresonance frequencies below 1930 MHZ and defining four first networkseries resonators having anti-resonance frequencies above 2155 MHz, anddefining the second plurality of acoustic wave resonators comprisesdefining three second network shunt resonators having motional resonancefrequencies between 1990 MHz and 2100 MHz and defining two secondnetwork series resonators having anti-resonance frequencies between 1990MHz and 2100 MHz.
 14. The method of designing a multi-band filter ofclaim 9, wherein the multi-band filter is a Band 1/Band 3 filter havinga lower pass-band of 1805 MHz to 1880 MHz and an upper pass-band of 2100MHz to 2170 MHz, defining the first plurality of acoustic waveresonators comprises defining four first network shunt resonators havingmotional resonance frequencies below 1805 MHZ and defining five firstnetwork series resonators having anti-resonance frequencies above 2170MHz, and defining the second plurality of acoustic wave resonatorscomprises defining three second network shunt resonators having resonantfrequencies between 1880 MHz and 2100 MHz and defining three secondnetwork series resonators having anti-resonance frequencies between 1880MHz and 2100 MHz.
 15. A communications device, comprising: a multi-bandfilter having a lower pass-band and an upper pass-band separated by anintervening stop-band, comprising: a first ladder network and a secondladder network coupled in series between a first port and a second port,wherein the first ladder network comprises two or more first-networkshunt resonators having respective motional resonance frequencies belowa lower edge of the lower pass-band, and two or more first-networkseries resonators having respective anti-resonance frequencies above anupper edge of the upper pass-band, and the second ladder network isconfigured to provide transmission zeros at frequencies within theintervening stop-band.
 16. The communications device of claim 15,wherein the second ladder network comprises: two or more second-networkshunt resonators and two or more second-network series resonators,wherein respective motional resonance frequencies of each of thesecond-network shunt resonators and respective anti-resonancefrequencies of each of the second network series resonators are withinthe intervening stop band.
 17. The communications device of claim 16,wherein each of the first-network series resonators, the first-networkshunt resonators, the second-network series resonators, and thesecond-network shunt resonators is a surface acoustic wave (SAW)resonator.
 18. The communications device of claim 17, furthercomprising: an antenna coupled to the first port of the multi-bandfilter; and a low noise amplifier coupled to the second port of themulti-band filter.
 19. The communications device of claim 18, whereinthe antenna, the multi-band filter, and the low noise amplifier are aportion of a diversity receiver within a cellular telephone.
 20. Amethod of designing a multi-band filter having a lower pass-band and anupper pass-band separated by an intervening stop-band, the methodcomprising: defining a first ladder network and a second ladder networkcoupled in series between a first port and a second port, the firstladder network including two or more first-network shunt resonators andtwo or more first-network series resonators; selecting respectivemotional resonance frequencies of the first-network shunt resonatorsbelow a lower edge of the lower pass-band; selecting respectiveanti-resonance frequencies for the first-network series resonators abovean upper edge of the upper pass-band; and defining the second laddernetwork to provide transmission zeros at frequencies within theintervening stop-band.
 21. The method of designing a multi-band filterof claim 20, wherein defining the second ladder network comprises:defining-two or more second-network shunt resonators and two or moresecond-network series resonators coupled to form the second laddernetwork; and selecting respective motional resonance frequencies of thesecond-network shunt resonators and respective anti-resonancefrequencies of the second-network series resonators within theintervening stop band.
 22. A Band 2/Band 4 filter having a lowerpass-band of 1930 MHz to 1990 MHz and an upper pass-band of 2100 MHz to2155 MHz separated by an intervening stop-band, comprising: a firstladder network and a second ladder network coupled in series between afirst port and a second port, wherein the first ladder network comprisesfour shunt resonators having motional resonance frequencies below 1930MHz and four series resonators having anti-resonant frequencies above2155 MHz, and the second ladder network comprises three shunt resonatorshaving motional resonance frequencies within the intervening stop bandand two series resonators having anti-resonance frequencies within theintervening stop band.
 23. A Band 1/Band 3 filter having a lowerpass-band of 1805 MHz to 1880 MHz and an upper pass-band of 2100 MHz to2170 MHz separated by an intervening stop-band, comprising: a firstladder network and a second ladder network coupled in series between afirst port and a second port, wherein the first ladder network comprisesfour shunt resonators having motional resonance frequencies below 1805MHz and four series resonators having anti-resonant frequencies above2170 MHz, and the second ladder network comprises three shunt resonatorshaving motional resonance frequencies within the intervening stop bandand two series resonators having anti-resonance frequencies within theintervening stop band.